1. Field of the Invention
This invention relates to a novel and improved process for preparing polymethylene polyphenyl polyisocyanates by a thermal decomposition of polymethylene polyphenyl polycarbamic acid esters obtained by a condensation reaction of N-phenyl carbamic acid esters with formaldehyde.
2. Description of the Prior Art
Isocyanates are highly useful substances for practical use as starting materials mainly for polyurethanes, and particularly, for example, toluene diisocyanates and polymethylene polyphenyl polyisocyanates have been commercially produced on a large scale.
These isocyanates are industrially produced by a reaction of a primary amine with phosgene. However the above process has many problems to be solved such as the toxicity of phosgene, the treatment of hydrochloride produced during reaction and the corrosive properties of the latter. Accordingly, the establishment of a successful process for producing isocyanates without using phosgene is highly sought, and various studies seeking such have been made. However such a successful process has not been found or developed.
As an example of an attempt to achieve such process is a process for the preparation of isocyanates by a thermal decomposition of carbamic acid esters. Attention has been riveted to the above process as an effective process for the preparation of isocyanates because a process for rather easily preparing carbamic acid esters directly from a nitro compound has recently been developed. However, the conventional thermal decomposition process has not been put to practical use because it has problems such as difficulties in the industrialization thereof and disadvantages incurred from the economical standpoint with respect to yield of isocyanates, reaction rate, materials for apparatus, temperature control, removal of by-products, and the like.
The greatest problem in the thermal decomposition of carbamic acid esters is that some unfavorable side reactions take place, and that isocyanates thus obtained are polymerized under reaction conditions for the thermal decomposition and react with carbamic acid esters, intermediate products, by-products or the like to form more complicated high-boiling by-products due to the high reactivity of such isocyanates. The formation of these high-boiling by-products is increased particularly in the case where polyisocyanates are prepared from polycarbamic acid esters and the by-product increase multiplies the problems.
The conventional process of the thermal decomposition of carbamic acid esters is classified into two major processes, one carried out in the gaseous phase at high temperatures and the other carried out in the liquid phase at relatively low temperatures. With respect to the first process, British Pat. No. 1,247,451 discloses a process in which the thermal decomposition reaction is carried out at a temperature of 400.degree. to 600.degree. C. in the presence of a Lewis acid. However, such process has disadvantages such as low yields, decomposition of catalyst at high temperature, corrosion of materials and an increased amount of polymer by-products. As an improved gaseous phase process over the above process, U.S. Pat. No. 3,870,739 discloses a process in which the reaction is carried out within 15 seconds of residence time at a temperature of 350.degree. to 550.degree. C. under reduced pressures. However, such process has the disadvantages that carbamic acid esters as starting materials have to be fed into a reaction site in the form of powder and that solid polymer by-products are also formed and accumulate within the reactor and condenser after the reaction is carried out for a long period of time to such an extent that continuous operation becomes difficult. Such process also presents great difficulties in being put into practical use, because the starting material must be supplied with a large amount of heat for the endothermic reaction required for the thermal decomposition in a short period of time.
On the other hand, many attempts have been made using a process for thermally decomposing carbamic acid esters in the liquid phase at a temperature comparatively lower than in the gaseous phase process. For example, U.S. Pat. No. 2,409,712 discloses a process in which N-lauryl carbamic acid ethoxyethyl esters are thermally decomposed under the conditions of 210.degree.-230.degree. C./2 mmHg in order to obtain the corresponding isocyanates at a yield of 75 percent. However, such yield is too low for such process to be put into practical use. Further, descriptions in J. Amer. Chem. Soc., 78, 1946(1956) and Bull. Chem. Soc., Japan 33 1137(1960) suggest that carbamic acid esters can be thermally decomposed in an inert solvent such as various hydrocarbons, ethers and nitrobenzene. However, the above references describe only the result of studies made concerning the reaction rate and teach nothing about isolation of isocyanates, yields of isocyanates and side reactions.
Recently processes such as the following have been suggested: a process in which carbamic acid esters are thermally decomposed at a temperature from 175.degree. to 350.degree. C. in the presence of a solvent such as hydrocarbons, ethers, ketones and esters, and the resulting isocyanates and alcohol are recovered separately as disclosed in U.S. Pat. No. 3,919,278; a process in which the thermal decomposition is carried out in the presence of a catalyst such as compounds of zinc, tin and the like in an inert solvent as disclosed in U.S. Pat. No. 4,081,472; and a process in which the thermal decomposition is carried out by the use of alkaline earth metal as catalyst in order to increase the yield of the product as disclosed in Japanese patent Laid-Open publication No. 88,201/1979. According to the above processes, yields of isocyanates are considerably increased, but rather large amounts of tarry high-boiling substances are still formed as by-products. Formation of such tarry high-boiling substances as by-products is considerably increased especially in the case where polyisocyanates are prepared by the thermal decomposition of polycarbamic acid esters. Such tarry high-boiling by-product substances are unseparable, are contained in the product and are responsible for such a deterioration of product quality that satisfactory polyurethanes are not produced from the product thus obtained.
The above processes are all relates to a process for the preparation of isocyanates, such as phenyl isocyanates, toluene diisocyanates and hexamethylene diisocyanates, which are recoverable by distillation after thermal decomposition and are consequently unsuitable for the preparation of a product containing high-boiling polyisocyanates, such as polymethylene polyphenyl polyisocyanates, which are unrecoverable by distillation under normal conditions.